[libc++] Remove some low value benchmarks (#175178)
This patch removes some of the benchmarks in our suite that are
relatively low value. Specifically:
- `lexicographical_compare` isn't benchmarked with every number of
elements between 1 and 8 anymore
- `bitset` isn't benchmarked with as many sizes anymore. Instead, only
some representative sizes are benchmarked.
- `to_chars` isn't benchmarked for every possible base anymore, but only
2, 8, 10, 16 and 23 (so we have an uncommon base as well)
